Apparatus and methods for edge filtering are known. As described in U.S. Pat. No. 4,664,814, a known edge filtering apparatus includes a number of filter sticks mounted within a pressure vessel divided into two compartments. Each filter stick includes a pervious tube, which is closed at one end. The closed end is disposed in a first compartment, into which fluid to be filtered is pumped under pressure. The opposite end is open, and in fluid communication with a second compartment for filtrate. A plurality of annular filter disks are stacked coaxially along the pervious tube. Pressurized fluid in the first compartment is forced through interstices between the filter disks into the tube and ultimately into the second compartment. The interstices trap particles to be filtered from the fluid.
In operation, the filter disks are compressed, such as by a spring or the like, along the axis of the tube. Axial compression of the stack controls the spacing between the disks so that particles will be trapped in the interstices between the disks during filtering, thereby preventing the particles from passing through to the tube. The more tightly the disks are pressed together, the smaller the particles that will be trapped between them. Where rough-surfaced disks are used, such as those described in U.S. Pat. No. 4,710,402, additional particles become trapped on the surfaces of the disks.
An interesting aspect of edge filtering is that filtering efficiency actually increases as more particles become trapped between or on the disks, because the trapped particles themselves help to filter additional particles. As particles build up, resistance to fluid flow through the filter increases, and fluid pressure builds up in the first compartment. The increased pressure further compresses the disks along the axis of the tube, further increasing resistance to fluid flow through the interstices. Under these conditions, the pump must force debris-laden fluid into the pressure vessel at increasing pressure to maintain a constant flow rate. Eventually, so many particles accumulate and compression of the filter disks is so great that efficiency begins to decrease, back pressure becomes undesirably high, and the filtering process must be shut down and the filter cleaned out, such as by reverse flushing.
It is not practical to increase the pump output pressure to maintain a constant flow rate through the edge filter. The pump output pressure is limited by the power of the pump, and as the maximum power limit is approached, the energy required by the pump becomes costly. Therefore, a way to maintain a constant flow rate through the filter, without stressing the pump, is needed.
In addition, known edge filtering methods can only be efficiently used with relatively low viscosity fluid. Even if the pump had enough power to initially force a high viscosity fluid through the conventional edge filter, the efficiency of the system would deteriorate rapidly because fluids of high viscosity compound the dynamics described above. Thus, a method of edge filtering high viscosity fluid is also needed.